Bipolar membranes are known to be useful for their ability to rectify alternating current, to improve desalination processes, to act as analogs of certain biological membranes, and to split water in the electrodialysis of acids and bases from salts. This latter property has great usefullness, since a wide range of soluble salts can be processed into commercially useful acids and bases by electrodialysis equipment employing bipolar membranes.
Bipolar membranes prepared by various procedures have been reported in the literature. For example, bipolar membranes have been prepared by adhering together, with heat and pressure or with an adhesive paste, two membranes consisting of oppositely charged ion exchange resins in an inert matrix (see U.S. Pat. No. 2,829,095). Anion and cation exchange membranes have also been fused together by means of heat and pressure to form bipolar membranes as disclosed in U.S. Pat. No. 3,372,101 and U.K. No. 1,038,777. The application of an anionic polyelectrolyte paste to a cation membrane which is then cured to yield a bipolar membrane has been described. Further, the preparations of bipolar membranes from a single sheet of polymeric material which is selectively functionalized on one side to give cation selectivity and on the other side to give anion selectivity has been disclosed in U.S. Pat. Nos. 3,388,080 and 3,654,125 and, more recently, U.S. Pat. Nos. 4,024,043, 4,057,481 and 4,140,815.
U.S. Pat. No. 4,116,889 describes bipolar membranes which exhibit good mechanical strength, ability to operate at high current density, high permselectivity, low potential drop and stable properties. As disclosed, a critical factor in the production of bipolar membranes exhibiting low potential drop is the degree of intimate contact between the anion and cation layers. The degree of contact must be controlled such that the layers do not interpenetrate each other to a degree which would result in a high resistance layer between the cation and anion permselective portions of the membrane. The bipolar membranes produced by the disclosed process have ion exchange resins dispersed in a polymer matrix having ionic groups of charge opposite the charge of ion exchange resin. The use of a polymer matrix containing crosslinked material (ion exchange resin) limits the degree of interpenetration to the extent necessary to produce quality bipolar membranes. An alternative process for producing bipolar membranes having an interfacial layer of ion exchange resin in a matrix polymer having ionic groups of charge opposite the charge of the ion exchange resin has been disclosed in EPA No 0,143,582. Not withstanding the disclosure of U.S. Pat. No. 4,116,889, the multitude of additional factors which contribute to improved stability, permselectivity and potential drop of dipolar membranes have not been heretofore uncovered.